1. Field of the Invention
The invention relates generally to resistivity logging of formations penetrated by a wellbore. More particularly, the invention relates to sensors that can be used to determine resistivity of mud in the wellbore.
2. Background Art
Oil and gas exploration requires a vast amount of information about the properties of the formations. Among various logging techniques, resistivity logging can provide useful information to locate hydrocarbon reservoirs. Various resistivity logging tools, including induction logging tools, are known in the art. One example of an induction logging tool may be found in U.S. Pat. No. 5,157,605 issued to Chandler et al. Various factors in the borehole may affect the accuracy of induction measurements. U.S. Pat. No. 5,041,975 issued to Minerbo et al. discloses a wellbore correction system for improving the accuracy of measurements made by induction logging apparatus. The correction system disclosed in Minerbo et al. corrects for errors arising from borehole effects, which may include effects due to currents flowing in the mud in the borehole, irregular shapes of boreholes, or tool eccentering. To correct for these borehole effects, it is often necessary to know the resistivity (or conductivity) of the mud in the wellbore. The mud conductivity often needs to be known with good accuracy because the mud conductivity may be orders of magnitude larger than the formation conductivity, and, therefore, errors of a few percent in mud conductivity may have a major impact on the accuracy of the determined formation conductivity.
To determine mud conductivities, various sensors have been used in the art, including an Auxiliary Measurement Sub (AMS sub). The AMS sub is typically located above an induction well logging apparatus and provides measurements of mud resistivity in the wellbore in addition to other measurements, such as temperature measurements and tool acceleration measurements. However, the AMS sub does not always provide accurate mud resistivity, due to potential influence of the formation resistivity.
U.S. Pat. No. 5,574,371 issued to Tabanou et al. discloses an improved mud sensor. This sensor is disposed on a probe that is connected to the bottom of a tool string. This sensor “looks” down instead of sideways to minimize the influence of the formation resistivity. This patent is assigned to the assignee of the present invention and is incorporated by reference in its entirety.
FIG. 1 shows a well logging apparatus including a mud sensor, such as those disclosed in the Tabanou et al. patent. As shown, a well logging apparatus 24 is disposed in wellbore 12. The well logging apparatus 24 may be any induction well logging apparatus, which may be a wireline tool or a logging-while-drill (LWD) tool. A mud resistivity sensor 26 is connected to the bottom 24a of the well logging apparatus 24. The sensor 26 is adapted to measure the resistivity, Rm, of the mud 18 below the sensor 26 in the wellbore 12.
FIG. 2 shows a detailed structure of sensor 26 of FIG. 1, as disclosed in the Tabanou et al. patent. As shown, the sensor 26 includes a current injector electrode (A0) 37 for emitting a current I0 into the mud 18, a current return electrode 34, which may be the conductive part of a downhole sub, for receiving the current I0 from the mud, and a pair of measurement electrodes (M1, M2) 36 for measuring the voltage drop between them. When a current I0 is emitted from the current injector electrode (A0) 37 and returned to the current return electrode 34, such current I0 propagates in the mud 18 in a direction approximately parallel to the longitudinal axis 24b of the well logging apparatus 24, when the current I0 passes in front of the measurement electrodes (M1, M2) 36. This provides measurements that are less influenced by the formation conductivity.
These prior art mud sensors use galvanic electrodes, which need to be in contact with the mud. These electrodes are exposed on the tool body and are typically insulated from the conductive tool body by insulating materials. The insulating material electrically isolates these electrodes from the conductive parts of the tool and also protects the electronics inside the tool from the harsh environment downhole. However, with the borehole being drilled deeper and deeper, the temperatures and pressures encountered by these tools increase accordingly, making it difficult for the insulating material to maintain good seal against the high pressures (up to 20,000 psi) and high temperatures downhole.
Therefore, there exists a need for better mud sensors and methods for mud resistivity measurements.